Associations of birth outcomes and air pollution at different time windows of pregnancy and neighborhood greenery
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Many studies suggest that environmental exposures, including air pollution, are associated with adverse birth outcomes. A growing body of research reports that neighborhood greenery is associated with reduced odds of adverse birth outcomes. We examine the associations between PM2.5, NO2, and O3 and preterm birth (PTB, gestational age < 37 weeks) and the potential mediation by tree canopy, a measure of neighborhood greenery. Outcome data were obtained from the North Carolina (NC) Department of Health and Human Services for the years 2003-2015 and limited to live, singleton births (N= 1315316). Daily air pollution concentrations at the Census tract level were obtained from hybrid ensemble models, averaged over each trimester and entire pregnancy, and assigned to participants based on residential address at date of birth. Tree canopy within 250m of each residence was calculated using 30m U.S. Forest Service Percent Tree Canopy Cover in 2011. We used logistic regression models to estimate the effects of air pollution on the odds of PTB with adjustment for confounders and then potential mediation effects by tree canopy. Analyses were also stratified to examine whether disparities in air pollution and greenery exist by race/ethnicity. Exposures to air pollution during the entire pregnancy had the strongest effects on PTB, with the highest magnitude association observed for PM2.5 (aOR: 1.024 [95% CIs: 1.021, 1.027]), a modest positive association observed for NO2, and a near-null association with O3. In addition, exposures at the third trimester tended to have greater associations with PTB compared to either of the first two trimesters. Tree canopy was associated with reduced odds of PTB (0.995 [0.992, 0.999]), and there was no evidence of mediation of air pollution effects. Associations of air pollution and greenery varied by race/ethnicity. Findings from this study suggest that environmental exposures during pregnancy are generally important for birth outcomes.